Nice addition to your story! It filled in the gap created by that scene quite nicely. Funny, but in your original story it didn't feel like it was missing. Now that I've read it, however, it is a strong scene that would add to the original story.

It doesn't seem to have occurred to you (and naturally would not have occurred to Lily, since she's unaware of it) that the Marauders don't need HER to spy on Snape. They have a Map that tells them where he is whenever they want to find him and an Invisibility Cloak that allows them to sneak up on him and take him by surprise whenever they choose or to just follow him around undetected. I strongly suspect they used the two as bullying aids, BTW ...